Linux 5.15 compat: standalone <linux/stdarg.h>

Kernel commits

39f75da7bcc8 ("isystem: trim/fixup stdarg.h and other headers")
c0891ac15f04 ("isystem: ship and use stdarg.h")
564f963eabd1 ("isystem: delete global -isystem compile option")

(for now can be found in linux-next.git tree, will land into the
 Linus' tree during the ongoing 5.15 cycle with one of akpm merges)

removed the -isystem flag and disallowed the inclusion of any
compiler header files. They also introduced a minimal
<linux/stdarg.h> as a replacement for <stdarg.h>.
include/os/linux/spl/sys/cmn_err.h in the ZFS source tree includes
<stdarg.h> unconditionally. Introduce a test for <linux/stdarg.h>
and include it instead of the compiler's one to prevent module
build breakage.

Reviewed-by: Tony Nguyen <tony.nguyen@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Alexander Lobakin <alobakin@pm.me>
Closes #12531

config/kernel.m4

@@ -133,6 +133,7 @@ AC_DEFUN([ZFS_AC_KERNEL_TEST_SRC], [
 	ZFS_AC_KERNEL_SRC_SIGINFO
 	ZFS_AC_KERNEL_SRC_SET_SPECIAL_STATE
 	ZFS_AC_KERNEL_SRC_VFS_SET_PAGE_DIRTY_NOBUFFERS
+	ZFS_AC_KERNEL_SRC_STANDALONE_LINUX_STDARG
 
 	AC_MSG_CHECKING([for available kernel interfaces])
 	ZFS_LINUX_TEST_COMPILE_ALL([kabi])
@@ -239,6 +240,7 @@ AC_DEFUN([ZFS_AC_KERNEL_TEST_RESULT], [
 	ZFS_AC_KERNEL_SIGINFO
 	ZFS_AC_KERNEL_SET_SPECIAL_STATE
 	ZFS_AC_KERNEL_VFS_SET_PAGE_DIRTY_NOBUFFERS
+	ZFS_AC_KERNEL_STANDALONE_LINUX_STDARG
 ])
 
 dnl #